The present invention relates in general to strain gauges, and in particular to a new and useful microbend fiber optic strain gauge which utilizes a coated optical fiber held and bent between corrugated plates, and a reference optical fiber which is exposed to the same thermal and other conditions but which is not held between the corrugated plates.
Strain gauges have been developed to measure structural loads to verify proper design of both individual components and the overall structure. Strain gauges now include foil, thin film, or wire resistance devices which are bonded or welded to the test piece to be measured. Loads applied to the test piece can cause it and the bonded gauge to extend, compress, or twist. The resulting strains induced in the gauge change its resistance. The gauge resistor forms one leg of a Wheatstone bridge. The bridge becomes unbalanced and a voltage developed in proportion to the amount of strain induced in the gauge. This approach is the basis of most strain gauge measurements performed today.
Difficulties are encountered when strain measurements are made at elevated temperatures, i.e. above 315.degree. C. For example, differential expansion between the gauge and test piece induces strain in the gauge, using up a substantial portion of its range and masking the load-induced strain to be measured. Furthermore, for accurate and reliable measurement, resistance strain gauges are generally limited to temperatures below about 315.degree. C. (about 600.degree. F.). Above this temperature, physical and metallurgical effects such as alloy segregation, phase changes, selective oxidation and diffusion result in large non-repeatable and unpredictable changes in the gauge output, and often in premature failure of the gauge or leadwire system.
Currently, no satisfactory method exists to perform accurate and reliable strain measurements at temperatures exceeding about 315.degree. C. A reliable, stable strain gauge is needed that will work at these elevated temperatures and which will match the thermal expansion of the test piece to enable the gauge to be bonded at low temperatures.
The measurement of the elongation of a structural member such as a long strut, presents several problems similar to those encountered in strain measurement. In a relatively benign environment which is free of vibration, the elongation may be slowly varying with time. This situation requires that an elongation sensor be capable of essentially D.C. measurements. As a consequence the sensor must exhibit extremely low drift.
This is further complicated when the structural member is in a hostile environment.
Instrumentation for in-flight monitoring of inlet and outlet engine conditions is needed for high-performance aircraft to improve fuel efficiency, engine performance, and overall reliability. This instrumentation must withstand the hostile engine environment which includes the high-temperature operating conditions and vibrations. Optical fibers and optical sensing methods have been applied to a number of measurements in hostile environments including displacement, velocity, strain, flow, temperature, particle size distribution, gas composition and fluorescence. These optical sensing methods can also be used to measure pressure in the hostile environment.
Optical sensors can also be designed to operate at high temperatures and in regions of high electromagnetic fields.